Coupled bandstop/bandpass filter

ABSTRACT

This disclosure includes a four pole bandstop filter coupled to a five pole bandpass filter tuned to an adjacent band to achieve a very steep attenuation curve between the two bands and an increased bandstop width.

United States Patent 1191 Boelter r a 1451 Feb. 26, 1974 [541 COUPLED BANDSTOP/BANDPASS FILTER 3,514,727 5/1070 Matsumoto 333/70 [75] Inventor: Donald A. oelter, ind anapolis, Ind 3,579,154 5/1971 Deen 333/76 [73] Assignee: General Aviation Electronics, Inc., FOREIGN PATENTS F APPLICATIONS Indianapolis, hid. 677,862 8/1952 Great Bntam 333/70 R 22 Filed: May 3 1971 OTHER PUBLICATIONS Henney K., Editor-in-Chief, The Radio Engineering 21 A .N 139 6 l 1 pp] 39 Hand Book, 3rd Edition, 1941, McGraw-H111 pp.

171-173 [52] US. Cl. 333/70 R, 333/77 [51] Int. Cl. H03h 7/08, H03h 7/38 Primary Examiner Rudolph v. Rolinec [58] Field of Search 333/70, 77 Assistant Examiner wm' Punter A11 A r, F W dard, W 'k 1, Em- [56] References Cited g'fgf f g en ar UNITED STATES PATENTS 1,227,113 5/1917 Campbell 333/70 R [57] ABSTRACT 1,882,631 /1932 3211111111111 333/77 x 4 7 2,092,709 53/1937 whedernm 333/70 R X Th1s disclosure 1nc1udes a four pole bandstop filter 2,247,898 7/1941 Wheeler 333/70 R X coupled to a five pole bandpass filter tuned to an adja- 3,530,408 9/1970 Brandon et a1. 333/70 R X cent band to achieve a very steep attenuation curve 3,316,510 4/1967 Poschenrieder 333/7 between the two bands and an increased bandstop 3,517,342 6/1970 Orchard 333/24 width 3,560,894 2/1971 Fettweis 333/72 1,493,600 5/1924 Campbell 333/70 1 Claim, 2 Drawing Figures 30 10\ I 1 37717371371 T.7 .7* F IF I u 11 1| 1 F II 1| 11 15 n 1 33 35 1 I I 33 1 f 1 =4 -4 4 1 12 14 10 1 I 32 34 35 7 i L;

. 1 v COUPLED BANDSTOP/BANDPASS FILTER BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to electrical filter networks for radio frequency signals.

2. Description of the Prior Art In the Alpha/200 aircraft radio manufactured by General Aviation, Inc., Indianapolis, Indiana, a conven tional S-pole capacitively coupled bandpass filterhas been used as a front end filter to pass an entire band of frequencies (108.0 to 127.9 MHz). To correct a problem of interference from FM broadcast stations, a two pole capacitively coupled bandstop filter was added between the receiver and the antenna. This two pole filter had a bandwidth of about 2 to 3 MHz and could be tuned to any FM frequency (105 MHz or below) depending upon the frequency of the most undesirable FM station in the area where the receiver was to be used. Due to the fact that aircraft fly in many areas and near many FM stations, this limitation in bandstop bandwidth limited the value of the bandstop filter in correcting the problem of FM broadcast station interference.

SUMMARY OF THE INVENTION BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an illustration of the preferred embodiment of the filter of this invention.

FIG. 2 is a block diagram illustrating variations of the invention.

' DESCRIPTION OF THE PREFERRED I EMBODIMENT Referring particularly to FIG. 1, there is illustrated a 4-pole bandstop filter 10 which couples to a 5-pole' bandpass filter 30. The 4-pole bandstop filter consists of four series resonant circuits (l1 and 12, 13 and 14, 15 and 16, and 17 and 18) coupled by capacitors 19, 20 and 21. The S-pole bandpass filter consists of five parallel resonant circuits (31 and 32, 33 and 34, 35 and 36, 37 and 38, and 39 and 40) coupled by capacitors 41, 42, 43, and 44. The connections to the bandpass filter 30 are made to taps on inductors 31 and 40 to obtain optimum coupling.

The bandpass filter 30 is tuned to pass the aircraft frequencies from 108.0 to 127.9 MHz, while the bandstop filter is tuned to stop FM frequencies from 88.1 to 107.9. The 4-pole bandstop filter 10 if tested alone would have a bandstop width of about 6 MHz, but when connected to the bandpass filter 30 the bandstop width increases to nearly 20 MHz and provides effective F M suppression over the entire FM band. The attenuation curve between the two bands becomes quite steep. 7

Variations in this circuit are possible without significant loss of the increased bandstop width and improved slope of the attenuation curve. FIG. 2 illustrates the minimum number of elements necessary to achieve the improved results. The bandstop filter connects to a bandpass filter 60 tuned to an adjacent band just as in FIG. 1. The bandstop filter includes 3 series resonant circuits 51, 52, and 53 coupled together by impedances 54 and 55 which may be resistive, capacitive, or inductive. The bandpass filter includes 2 parallel resonant circuits 61 and 62 coupled together by impedance 63 which may be resistive, capacitive, or inductive. Although the resonant circuits illustrated herein are made with inductors and capacitors, it is intended that equivalently functioning items such as cavity resonators or crystals be encompassed in the term resonant circuit, it being the effect which is important.

I claim: 7 1. An electrical ladder-type filter which comprises a. a bandstop filter having four series resonant circuits in transverse branches and connected to a common point, and capacitors to couple between pairs of series resonant circuits, and

b. a bandpass filter coupled to said bandstop filter and having 5 parallel resonant circuits in transverse branches, said bandpass filter having a capacitor to couple between two of the parallel resonant circuits, said bandstop filter being tuned to a band adjacent to the passband of said bandpass filter, and wherein one of said parallel resonant circuits includes a tapped inductor and the bandstop filter connects to the tap on said tapped inductor. 

1. An electrical ladder-type filter which comprises a. a bandstop filter having four series resonant circuits in transverse branches and connected to a common point, and capacitors to couple between pairs of series resonant circuits, and b. a bandpass filter coupled to said bandstop filter and having 5 parallel resonant circuits in transverse branches, said bandpass filter having a capacitor to couple between two of the parallel resonant circuits, said bandstop filter being tuned to a band adjacent to the passband of said bandpass filter, and wherein one of said parallel resonant circuits includes a tapped inductor and the bandstop filter connects to the tap on said tapped inductor. 